1. Field of the Invention
This invention relates to a high heat-resistant catalyst support used for a catalyst for purifying automotive exhaust gases and the like, a method of producing the catalyst support at a low cost, a high heat-resistant catalyst using the catalyst support, and its production method.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NO.sub.x) to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat-resistant supporting base material formed of cordierite, a porous catalyst support layer formed of gamma-alumina and disposed on the supporting base material, and a noble metal catalyst ingredient such as platinum (Pt) and rhodium (Rh) loaded on the porous catalyst support layer. Further, the 3-way catalysts are also known in which ceria i.e., cerium oxide having an oxygen storage ability is employed in addition to the above catalyst ingredient in order to improve catalytic activity at low temperatures.
In the meanwhile, carbon dioxide (CO.sub.2) in exhaust gases from internal combustion engines of automobiles and the like has become a problem in view of global environmental conservation. Lean burn in oxygen excessive atmospheres is desired as a means of dissolving the problem. The lean burn improves fuel consumption, and as a result the amount of fuel used is decreased and CO.sub.2, which is included in combustion exhaust gas, can be suppressed from generating.
In this respect, the conventional 3-way catalysts aim to oxidize CO and HC and reduce NO.sub.x simultaneously into innocuous entities when the air-fuel ratio is at the stoichiometric point (or the ideal air-fuel ratio), and cannot exhibit sufficient reduction and removal of NO.sub.x in the exhaust gases which contain oxygen in excessive amounts required for oxidizing carbon monoxide and hydrocarbons therein at the time of lean burn. Hence, it has been desired to develop catalysts and exhaust gas purifying systems which are capable of adequately purifying NO.sub.x even in oxygen excessive atmospheres.
In view of the aforementioned circumstances, the applicants et al of the present invention have proposed an exhaust gas purifying catalyst in which alkaline-earth metals and platinum (Pt) are loaded on a porous support formed of alumina and the like in Japanese Unexamined Patent Publication (KOKAI) No.5-317,652, an exhaust gas purifying catalyst in which lanthanum (La) and platinum (Pt) are loaded on a porous support in Japanese Unexamined Patent Publication (KOKAI) No.5-168,860, and an exhaust gas purifying catalyst in which alkali metals and platinum (Pt) are loaded on an alumina support in Japanese Unexamined Patent Publication (KOKAI) No.6-31,139. In using these catalysts, NO.sub.x are adsorbed on oxides of alkaline-earth metals or lanthanum oxide on the fuel-lean side (i.e., in the oxygen excessive atmospheres), and the adsorbed NO.sub.x react with reducing components such as HC and CO at the stoichiometric point or on the fuel-rich side (i.e., in the oxygen-lean atmospheres). So, these catalysts attain excellent NO.sub.x purifying performance even on the fuel-lean side.
The production of these catalysts is mainly achieved by what we call a loading-by-water absorption method. For example, first, a porous support formed of alumina or the like is impregnated with a solution containing a chemical compound of a noble metal catalyst ingredient, dried, and calcined, thereby loading the noble metal catalyst ingredient thereon. Second, the noble metal-loaded support is reimpregnated with a solution containing a chemical compound of an NO.sub.x storage component, dried, and calcined, thereby loading the NO.sub.x storage component thereon.
By the way, the average temperature and the maximum temperature of inlet gases to the bed of catalysts tend to rise more and more in recent years due to severe restrictions on exhaust gases and the improvement in the capacity of engines. Therefore, it is desired to make a further improvement in the heat resistance of catalysts for purifying exhaust gases. Further, with an increase in inlet gas temperatures, it is also desired to improve NO.sub.x conversion at elevated temperatures.
In the conventional catalysts, however, there arises a problem that NO.sub.x storage components react with catalyst supports at high temperatures and as a result the NO.sub.x storage ability of the NO.sub.x storage components is deteriorated. Besides, in the conventional catalysts, a temperature range in which the maximum catalyst performance is obtained, namely, a temperature window is narrow, and the NO.sub.x conversion is hardly secured at elevated temperatures.
Further, in the conventional catalysts, the NO.sub.x storage components are poisoned by SO.sub.x which are produced from a very small amount of sulfur contained in fuel, in other words, the NO.sub.x storage ability is decreased due to sulfate generation. Consequently, the catalysts are degraded in durability.
Further, in the conventional methods of producing catalysts, the NO.sub.x storage components are loaded by a loading-by-water absorption method. When this method is employed, the NO.sub.x storage components have a poor dispersion, i.e., a maldistribution. As a result, crystallization of the NO.sub.x storage components is promoted at and around a part having a high loading concentration of the NO.sub.x storage components, so that the NO.sub.x storage ability is deteriorated. The NO.sub.x storage ability, especially at elevated temperatures is greatly influenced by the combination of NO.sub.x storage components and catalyst supports, and the dispersibility of NO.sub.x storage components.
Further, when the NO.sub.x storage components are poorly dispersed, sulfate crystals generated by sulfur poisoning easily grow and therefore, become more difficult to be removed, so that the durability of the catalysts is decreased. In addition, in the case where the NO.sub.x storage component is an alkali metal, the NO.sub.x storage component tends to be flown away or eluted by water vapor contained in exhaust gases, because the NO.sub.x storage component is loaded on the surface of the catalyst support in the conventional methods. Hence, the catalyst is poor in durability.